Field of the Invention and Related Art Statement
The present invention generally relates to a technique for curing or vulcanizing rubber or plastic articles and more particularly to a method of measuring a temperature at a point within a rubber or plastic article during a vulcanization. This invention also relates to a method of controlling the vulcanization of a pneumatic tire.
In the pneumatic tire manufacturing process, a green tire is placed in a vulcanizing machine and is subjected to heat and pressure. In order to manufacture a tire having desired characteristics, it is important to control the vulcanizing process in an accurate and reliable manner. In the known curing process the temperature of a vulcanizing mold and temperature and pressure inside a bladder are kept constant and heating and pressing are carried out for a predetermined time period. In practice, temperatures at various points within the tire vary due to variations of various factors such as variations in construction of green tires, mold temperature and bladder temperature and pressure. These variations may fluctuate day by day as well as season by season. Therefore, in the known method, in order to avoid undesired variations under vulcanization, the curing time period has to be set longer than may be necessary. Then, undesired over cure might occur and the desired characteristics of tire could not be attained. Moreover, the longer curing time reduces the manufacturing efficiency and requires a greater amount of heating fluid.
In order to mitigate the above mentioned drawback, there have been proposed various methods of controlling the vulcanizing process. For instance, in U.S. Pat. No. 3,819,915, there is disclosed a method of controlling the vulcanization, in which a temperature at a point within a tire set in the vulcanizing machine is detected by selectively inserting a thermometer into the tire and the vulcanized condition of the tire is judged on the basis of the detected temperature. However, in such a method since the thermometer is inserted into the tire, a trace of the thermometer insertion remains in a surface of the vulcanized tire and the commercial value of tire might be decreased. Further, it is rather difficult to measure a true temperature of the tire in a precise manner by inserting the thermometer into the tire. Moreover, since only a temperature at the predetermined point within the tire can be measured, a temperature at a point where the vulcanization has been effected to the least extent (point of least vulcanization) could not be detected, so that the vulcanization could not be controlled precisely. It is apparent that the operation of inserting and pulling the thermometer into and from the tire is very cumbersome and the thermometer might be broken easily. Further in this known method since the thermometer is inserted into the outer surface of the tire, it is impossible to detect temperatures at various points within the tire.
In U.S. Pat. No. 3,649,729 there is described another known method of controlling the vulcanization. In this method, a temperature at a boundary between the tire and mold and a temperature at boundary between the tire and bladder are measured and temperatures at points within the tire are calculated from the above detected temperatures at the boundaries, and then vulcanization of tire is estimated from the calculated temperatures.
In this known method, it is essential to use temperature sensors locatable at the boundaries between the tire and the mold and bladder, respectively. However, in practice, it is almost impossible to locate the temperature sensor at the boundary between the tire inner surface and an outer surface of the bladder. If use is made of an expansible bladder made of rubber, the temperature sensor could never be applied onto the outer surface of the bladder. Although the temperature sensor may be secured on a foldable bladder, the bladder has to be exchanged by new one after using it only for a few days, so that the temperature sensor would be also wasted. Therefore, such a method could not be performed economically. Further the maintenance of the temperature sensor would be very cumbersome.
In the above mentioned known methods, the degree of tire vulcanization is estimated on the basis of the well-known Arrhenius equation, while the point of least vulcanization is assumed to be remained at a fixed point. However, in practice, the point of least vulcanization varies during the curing process, so that the vulcanization could not be detected in an accurate manner.
In U.S. Pat. No. 4,371,483, there is disclosed still another method of controlling the vulcanization. In this method, a temperature profile within the tire is calculated by measured temperatures and a point of least vulcanization is derived from the temperature profile. Therefore, the point of least cure can be traced during the vulcanization. However, in this method, temperatures at the boundaries between the tire and the mold and bladder could not be measured accurately. That is to say, the temperature at the boundary between the mold and the outer surface of the tire is estimated by measuring a temperature on an outer surface of the mold. However, the temperature on the outer surface of the mold does not reflect the temperature at the outer surface of tire, because the temperature on the outer surface of mold would be varied in accordance with an ambient temperature. Further, in this known method, the temperature at the boundary between the inner surface of the tire and the bladder is estimated from or temperature at a supply or drain pipe for introducing or discharging the heated fluid into or out of the inside of the bladder. However, the temperature at the pipe outside the bladder could never represent the temperature at the boundary between the tire and the bladder. Particularly, in case of using steam and/or gas as the heating fluid medium passing through the bladder, the temperature at the boundary between the inner surface of tire and the bladder could never be estimated from the temperature at the pipe connected to the bladder.
Therefore, in the known method, the temperatures at points within the tire could not be estimated in an accurate manner, so that vulcanization could not be controlled precisely.
Further, in the method disclosed in the above mentioned U.S. Pat. No. 4,371,483, the state of cure at a tire shoulder is calculated by a finite element method using a finite number of rules and steps. However, in the finite element method, a plurality of coefficients have to be calculated by using a large scale computer for respective kinds of tires to be cured, so that the process becomes quite complicated and cumbersome. Moreover, the point of least vulcanization is not always existent at the shoulder, so that this method could be applied only to limited kinds of tires.